The invention relates to a friction vacuum pump with a stator and a rotor, which form at least two pump stages with one gas inlet each, as well as junction means for the pump stages, which are equipped with junction openings and serve for connecting the gas inlets of the pump stages with devices to be evacuated.
A friction vacuum pump of this type is known from DE-A-43 31 589. It serves preferably for evacuating particle beam apparatus (for example mass spectrometers) with chambers separated from one another by diaphragms, in which different pressures are to obtain during operation of the particle beam apparatus. It is known per se to use separate vacuum pumps for generating these pressures.
DE-A-43 31 589 discloses generating with the aid of only one vacuum pump system the different pressures required by the particle beam apparatus. The pump system comprises two turbomolecular and one molecular (Holweck) pump stage. These pump stages are disposed such that one axially succeeds the other. Each pump stage comprises a gas inlet (front-side gas penetration area), which, via junction means, is connected with the associated chamber of the device to be evacuated. In the solution according to DE-A-34 31 589 the housing itself and a laterally disposed auxiliary housing serve as junction means. The housing itself is equipped with a front-side junction opening for connecting the gas inlet of the first pump stage with the device to be evacuated. In the auxiliary housing are provided connection lines which connect the associated inlets of the further pump stages with further junction openings. These are each connected, in turn, with the associated chambers in the device to be evacuated. Since the junction openings in the auxiliary housing are located in a common plane (perpendicularly to the rotor axis) with the junction opening of the first pump stage, the connection lines located in the auxiliary housing, must be relatively long. Thereby relatively large conductance losses in the connection lines result, which is in particular of disadvantage if a high suction capacity is desired precisely in the region of an intermediate junction.
The present invention is based on the task of implementing a friction vacuum pump of the above described type such that the suction capacity of the intermediate stages is not impaired by high conductance losses in connection lines.
This task is solved according to the invention thereby that the junction openings are located in a plane laterally adjacent to the pump stages such that the spacing between the junction openings and the rotor axis is of minimum feasible size.
These measures ensure that the spacing between the particular gas inlet of the intermediate stages and the associated junction openings is also of minimum feasible size. Conductance losses are low. The suction capacity active in the region of the gas inlet of all pump stages is available nearly unchanged even in the region of the associated junction openings.
While realization of the measures according to the invention leads to the fact that the gases to be transported in the inlet region of the first pump stage, thus exactly at that site at which the pressure is lowest, must be deviated, however, the loss in conductance caused thereby can be kept low since the spacing between the gas inlet and the plane of the junction opening still is relatively small and, in addition, nothing stands in the way of selecting in this region a greater diameter. Moreover, for the majority of applications especially high values for the suction capacity are not demanded in the region of the inlet of the first (high-vacuum side) pump stage. There is frequently even the necessity to reduce the suction capacity at this site. It is the essential purpose of the first pump stage to ensure a high compression ratio. The blade properties (number of turbo stages, blade spacing, angle of inclination etc.) must be designed with this in mind. Essential is the separation of the two working pressure regions of the two pump stages. As a rule, high suction capacity is only required at the intermediate inlet(s). This goal can also be attained through the selection of special blade geometries. Applying the measures according to the invention ensures precisely in this region that losses in suction capacity are largely avoided.
Critical for the suction capacity of a pump stage is the accessibility of the gas molecules to the gas inlet (effective gas penetration area). In order to attain this goal, it is known to provide in an intermediate stage a greater spacing between the preceding stage and its gas inlet. It is especially advantageous if this spacing is at least one fourth, preferably one third, of the diameter of the rotor.